On Monday night, a passenger boarded United Flight 1284 from Houston Intercontinental to New York – LaGuardia with a small dog inside a TSA-compliant carrier. According to the passenger, a flight attendant then demanded that the carrier and animal be placed in the overhead bin for the duration of the flight, instead of under the seat, as is common practice. A witness wrote on Facebook that the passenger protested, but eventually complied. The dog then died sometime during the flight, according to The Points Guy.

Per United’s in-cabin pet policy, “A pet traveling in cabin must be carried in an approved hard-sided or soft-sided kennel. The kennel must fit completely under the seat in front of the customer and remain there at all times.” As such, people traveling with an in-cabin pet cannot be seated in an emergency exit or bulkhead row.

In a statement, United said, “This was a tragic accident that should never have occurred, as pets should never be placed in the overhead bin. We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them. We are thoroughly investigating what occurred to prevent this from ever happening again.”

Pet deaths in the cabin are rare. But as previously reported by Traveler‘s Rachel Rabkin Peachman, dozens of animals died flying in cargo on U.S. airlines in 2016. In April 2017, Simon, a three-foot-long, ten-month-old Continental Giant rabbit, was found dead in the cargo section of a Boeing 767 upon arrival at Chicago O’Hare.